The present invention relates to a control valve for vehicle brake systems having two brake circuits, wherein the full operating pressure is led into the one (first) of the two brake circuits when the pressure drops in the other (second) of the two brake circuits. The control valve incorporates a pressure reducing valve arranged in the pressure medium connection from the master cylinder to the first circuit and effects the control, the pressure reducing valve including at least one stepped piston and one sensing piston and in which the pistons are arranged in a common bore and are moveable towards one another.
Control valves of this type and of similar types are known, e.g. from German Pat. DE-OS No. 24 27 506, in which, however, the sensing part for determination of the pressure drop is also actuated by the controlled pressure in the first circuit and is accordingly dependent on the respective state of control. Furthermore, this known valve does not guarantee the movement of all seals in an efficient brake.